Various types of transvenous pacing and cardioversion/defibrillation leads have been developed for endocardial introduction into different chambers of a patient's heart, typically the right ventricle (RV) or right atrial (RA) appendage, as well as the coronary sinus (CS). These flexible leads are usually constructed with an outer polymer insulator sheath, such as a flexible silicone or polyurethane tube or coating for encasing one or more coiled or cabled wire electrical conductors. One such conductor is typically attached at its distal tip to the shank portion of a tip electrode. In bipolar or multipolar leads, one or more coiled wire conductors are provided in a coaxial or co-linear relation to a first coiled wire conductor, and are connected to electrodes situated along the lead body. The proximal ends of the conductors are coupled to a connector which includes a single pin in unipolar leads and additional pins or rings in bipolar and multipolar leads.
The tip electrode is usually placed in contact with the myocardial tissue by passage through venous access, often the subclavian or cephalic vein or one of its branches, which leads to the endocardial surface of the heart chambers. The tip electrode may be held in place passively by silicone or polyurethane tines within the trabeculae of the RV apex, as described, for example, in U.S. Pat. No. 3,902,501 to Citron et al. The tines or fins extend outwardly and are usually molded separately and bonded onto the distal end of the lead proximal to the tip electrode. Alternatively, the tip electrode may be held in place actively through the use of a manipulated anchor or screw that penetrates the myocardium, as described, for example, in U.S. Pat. No. 3,974,834 to Kane. These fixation mechanisms help to prevent dislodgment of the tip, thus maintaining consistent sensing and pacing characteristics over time.
Although the state of the art of implantable pulse generators and endocardial lead technology has advanced considerably, endocardial leads nevertheless occasionally fail for a variety of reasons, such as insulation failure, sensing failure, wire conductor fracture, or an increase in electrode resistance beyond a desirable level. Also, in some instances, it may be desirable to add one or more leads to stimulate different portions of the heart than are presently being stimulated with leads already in place. Many patients have one or more, and sometimes as many as four or five previously used (abandoned) and currently used leads in their veins and heart.
The risks of removing leads or introducing additional leads in the heart and venous system include infection, physiological complications, obstruction to blood flow, and formation of blood clots which may embolize to the lung and produce severe complications and even death. In addition, extra leads in the heart can interfere with tricuspid valve and mechanical function, and can cause considerable difficulty in the positioning and attachment of new endocardial leads in the heart.
Typically in the first few months of lead implant, fibrotic tissue encapsulates the lead, especially in areas touching biological tissue, such as the endothelial layer of veins, valves, heart wall, and trabeculae. When small diameter veins through which the lead passes become occluded with fibrotic tissue, separating the lead from the vein becomes difficult and may severely damage the vein or tissue attached to the lead, making lead explant very risky, difficult and oftentimes dangerous.
Several attempts have been made to alleviate the lead explantability problem by using device or tool-assisted methods for removing the lead. A few exemplary lead removal techniques are described in the following publications:
U.S. Pat. No. 4,574,800 to Peers-Trevarton teaches a lead extractor which is inserted into the lumen of an implanted lead and wedged against its inner structure (typically a coil) at a distal location, such as near an electrode implanted in the atrium or ventricle. This wedging condition permits a pulling force to be transmitted along the length of the extractor to the implanted electrode location.
U.S. Pat. No. 5,231,996 to Bardy et al. describes an endocardial lead having a structure that strengthens the lead body to enable its removal by traction after a period of chronic implant. One or more relaxed, nonextensible filaments are loosely contained within the insulating sheath, and have their proximal and distal ends mechanically connected to the connector and electrode shank of the lead. These filaments operate as means for allowing the lead body to reach a stretched length exceeding the relaxed length by an amount sufficient to allow the lead to be stretched without breaking during removal by traction.
U.S. Pat. No. 5,207,683 to Goode et al. teaches the use of a flexible stylet wire with an expandable wire coil attached to the distal end for engaging the coiled structure of the lead.
A potential problem with the above intraluminal devices is that they do not address the possible encapsulating fibrous tissue attached to the lead, either along the lead body or at the distal end, especially with tines. When using these intraluminal extraction devices, the tissue may tear instead of releasing from the lead, causing atrial or ventricular avulsion, tears in the vein or heart, tamponade, and/or hemothorax. Immediate surgical intervention requiring a thoracotomy may be necessary to prevent death.
Other extraction devices have been developed that are meant to fit over the lead to separate the lead from tissue attachments. Such devices are meant to work either alone or in conjunction with intraluminal devices, such as the locking stylets described above. One company that makes such lead extraction tools is Cook Pacemaker Corporation (Leechburg, Pa). However, in many cases, these sheaths are incapable of easily separating calcified tissue from the lead. Often, a femoral removal approach is needed, especially as lead implant duration increases. It is preferred that the leads be removable by the venous implant site rather than femorally, if possible. These extraction sheaths, used alone or in combination with locking stylet devices, do not overcome the risks described above.
In addition to providing special tools to aid in lead extraction, the lead itself may be designed to be more easily extracted, such as by adding coatings, discouraging tissue ingrowth, and making the lead isodiametric. Coated leads and catheters, as well as coating materials are described in the following publications, all of which are incorporated herein by reference:
U.S. Pat. No. 4,487,808 to Lambert describes the process of coating a polymer surface with a hydrophilic coating with low friction under wet conditions. The process comprises applying to the polymer surface a solution containing between 0.05 and 40% of a compound which comprises at least two unreacted isocyanate groups per molecule, evaporating the solvent, applying a solution containing between 0.5 and 50% of polyethylene oxide to the thus treated polymer surface, evaporating the solvent of the latter solution, and curing the coating at elevated temperature. This patent aims at facilitating the insertion of medical instruments inside a body cavity by decreasing the coefficient of friction of the surface of the device or lead.
U.S. Pat. No. 5,041,100 to Rowland et al. describes a catheter to which a friction-reducing coating may be applied, to reduce catheter friction particularly when the coating is hydrated. The coating includes a mixture of a structural plastic material and high molecular weight polyethylene oxide, for facilitating the insertion of the catheter into a patient's body.
U.S. Pat. No. 5,077,352 to Elton describes still other abrasion resistant, hydrophilic, lubricious organic coatings for application to the outer surfaces of inorganic materials or organic polymeric medical devices, to facilitate the introduction of these devices inside the patient's body.
However, while the foregoing coating processes may reduce friction to aid in implanting the leads, none of them were developed for nor adequately addresses the problem of explantation of leads should adhesions develop on the surface and become firmly attached. It appears that in prior art only a thin, surface coating is applied, with no significant dimensional changes. Only a thin coating is necessary to reduce friction to facilitate ease of insertion.
Hydrogel coatings have been proposed for other uses in pacing and defibrillation leads. The following are such examples, and are incorporated herein by reference:
European patent application No. 057,450 to Cahalan et al. relates to a body implantable lead having a polymer-based gel electrode. European patent application No. 057,451 to Juncker et al. relates to a body implantable lead having a pressure-cushioned electrode. In both cases, the pacing tip electrode is coated with a hydrogel, thus separating the solid electrode from the excitable tissue and increasing the effective electrode area. While increasing effective electrode area is desirable for defibrillation in which the goal is field stimulation, it is desirable to keep effective pacing electrode area small to minimize pacing thresholds. Therefore, increased pacing thresholds would be a disadvantage for the Cahalan et al. and Juncker et al. inventions.
1995 NASPE abstract 452, entitled "A New Surgical Temporary Pacing Lead: Easy to be Fixed and Easy to be Removed," by Yokoyama et al. describes the use of a material composed of absorbable polyglycolic acid felt (PGA felt) for facilitating the extraction of temporary pacing leads seven to ten days after implantation. However, there is no clear indication that this material can be useful to resolve fibrotic encapsulation problems resulting from the long term implantation of the lead, since the PGA felt is absorbed by the tissue after ten days, and tissue encapsulation may then begin. Additionally, the use of the PGA felt seems to be limited to the distal end of the lead, and consequently neglects the fibrotic growth on the defibrillation electrode and lead body.
U.S. Pat. No. 5,020,544 to Dahl et al. describes a defibrillation patch electrode having a hydrogel incorporated in the porous conductive screen for preventing tissue ingrowth. The hydrogel can serve as a drug reservoir for antibiotics, antiseptics, antiarrhythmics, or anti-inflammatory steroids.
Polymer hydrogels and other coatings have also been recommended for use on implantable devices to stimulate the attachment of endothelial cells for improving thromboresistance. U.S. Pat. No. 4,836,884 to McAuslan describes such hydrogels.
U.S. Pat. No. 5,090,422 to Dahl et al. describes a porous implantable enclosure that covers and isolates an electrode in a way which allows electrical coductivity via bodily fluid which passes through but separates the electrode from the adjacent tissue in the manner of a dissection plane which substantially prevents tissue ingrowth. In addition to the improved explantability, the porous covering is also intended to reduce tissue burning and edema. However, the porous implantable enclosures in Dahl are not described as expandable, so that the lead would have to be implanted with the covering already at the final thickness required to provide the desired effects, which is disclosed as 10 to 100 mils. This would increase the overall diameter of the lead by 20 to 200 mils (up to about 5 mm), which would substantially increase the introducer size needed and reduce maneuverability through the vein to the implant sight.
While the foregoing coating processes may aid in preventing tissue ingrowth into the interior of the leads, none of them adequately addresses the problem of explantation of leads should adhesions develop on the surface and become firmly attached. It appears that only a thin, surface coating is applied, with no significant dimensional changes.
None of the above solutions completely satisfies the need for a new method and lead structure which is easy to implant and improves the rate of successful lead explantation, particularly after the encapsulated tissue has rendered various traction removal methods impractical.